A conventional electric vehicle (EV) uses only an electric motor as a travel drive power source. This electric vehicle is a motor-driven vehicle, in which a travel-use battery for supplying electric power to the motor is charged with electricity supplied from an electric power supply source external to the electric vehicle. A conventional hybrid vehicle (HV) uses both an electric motor and an internal combustion engine as a travel drive power source. This hybrid vehicle is also a motor and engine-driven vehicle (plug-in hybrid vehicle PHV), in which a travel-use battery is charged with electricity supplied from an electric power supply source external to the hybrid vehicle.
In charging the battery of vehicle, for example, the battery is charged by electrically connecting a motor-driven vehicle to a charging device such as an exclusive charging station. It is proposed recently to monitor the charge state of the battery of the vehicle at a location away from the charging device and the vehicle.
The following patent document proposes one technology for monitoring a charge state of a battery mounted on an electric vehicle at a location remote from the electric vehicle. According to this technology, the electric vehicle is provided with an in-vehicle communication device, which transmits charge information of the battery through a mobile device packet communication network to a charge management station located at a remote place. The charge information of the battery of the electric vehicle is thus acquired at the remote location away from the electric vehicle. The patent document 1 further proposes another technology for managing charge states of a plurality of electric vehicles. According to this technology, the in-vehicle communication device is provided in each of the plurality of electric vehicles and communicates with the charge management station.    Patent document: JP 2002-123888A
According to the technology proposed by the patent document, the charge management station communicates with each of the in-vehicle communication devices of the electric vehicles. It is therefore necessary to perform the communication through the mobile device packet communication network as many times as the number of the electric vehicles, the charge states of the batteries of which should be monitored. This results in an increase in the amount of communication traffic.
Further, according to the technology proposed by the patent document, the communication between the charge management station and each in-vehicle communication device of the electric vehicles is performed through the mobile device packet communication network, which is toll-charged. This also results in an increase in the amount of communication costs in correspondence to the increased in the amount of communication traffic.
In addition, according to the technology proposed by the patent document, the communication is performed between one charge management station and the plurality of in-vehicle communication devices of the electric vehicles. This results in an increase in the load of communication processing at the charge management station.